Subcritical crack growth and long-term strength in rock and cementitious material

High-strength and ultra low-permeability concrete (HSULPC) is a strong candidate for a radioactive waste package containing transuranic radionuclides (TRU waste) for geological disposal. Knowledge of the time-dependent fracturing of HSULPC and surrounding rock mass is essential to assess the long-term stability of such underground repositories. We have measured crack velocity in andesite and HSULPC both in air and water to examine subcritical crack growth by the Double-Torsion method. In air, the crack velocity in andesite increased when the temperature and relative humidity increased. On the other hand, the temperature and relative humidity had little effect on the crack velocity in HSULPC in air. In water, the crack velocity increased when the temperature was higher for both andesite and HSULPC. Using these experimental results, the long-term strength was estimated. It was shown that the long-term strength of HSULPC was higher than that of andesite. In air, the long-term strength for andesite was affected by the temperature and relative humidity. The long-term strength for andesite decreased when the temperature or relative humidity increased. For HSULPC, the change of the long-term strength with varying temperature or relative humidity was smaller than andesite in air. In water, the long-term strength for both materials decreased with increasing the temperature. Comparing the long-term strength of andesite and HSULPC at the same environmental conditions, it was recognized that the decrease of the long-term strength of HSULPC is smaller than that of andesite. The long-term strength in water was smaller than that in air for both materials.     

Long-term behavior of existing low-strength reinforced concrete beams strengthened with carbon fiber composite sheets

This paper presents the results of an experimental study of the short- and long-term behavior of low-strength reinforced concrete (RC) beams strengthened with carbon fiber reinforced polymer (CFRP). A numerical analysis model was developed and verified for the calculation of the long-term deflection and maximum allowable long-term load of such RC beams. A parametric study was also conducted and it was found that the maximum allowable long-term load of a CFRP-strengthened beam was dominated by the deflection of RC beam when the cubic compressive strength of concrete was less than a certain value. For concrete of higher strength, the maximum allowable long-term load was dominated by the stress levels of the steel bars. It was also found that the yielding load of the strengthened beams increased significantly with areas strengthened by CFRP sheets and steel bars, while the maximum allowable long-term load only increased slightly.     

Evaluating long-term service performance under short-term forecast updates

Managing product availability in a cost-effective way has always been a major challenge faced by inventory managers. We study the problem of a firm selling a perishable product with short-term demand patterns and a long-term service target using the newsvendor framework. The newsvendor determines his long-term order at the first stage, and revises the order according to a short-term forecast update at the second stage. He also evaluates a long-term service target for his overall performance on product availability across all possible forecast updates. We characterise his optimal inventory policy that minimises the expected inventory cost while meeting the long-term service target. Both in-stock rate and fill rate targets are examined.     

The influence of quasi-static loading regimes on the strength of vessels operating under pressure

The influence of long-term quasi-static loading on characteristics of deformation stress and fracture of vessels operating under pressure is examined. It is proposed to account for parameters of accumulated stress in order to estimate the state of material in these conditions. The physicochemical properties of material connected with different regimes of static, long-term, and cyclic quasi-static impacts which are used for estimating limit states of equipment working in conditions of long-term quasi-static loading are studied.     

Challenges of long-term vascular access in pediatric hemodialysis: Recommendations for practitioners

Kidney transplantation is the preferred treatment of end-stage renal disease in children. However, time to transplant varies, making a well-functioning long-term vascular access essential for performing hemodialysis efficiently and without disruption until a kidney becomes available. However, establishing long-term vascular access in pediatric patients can present distinct challenges due to this population's unique characteristics, such as smaller body size and lower-diameter blood vessels. There are three main pediatric long-term vascular access options, which include central venous catheters (CVC), arteriovenous fistula (AVF), and arteriovenous graft (AVG). CVC are currently the most widely used modality, although various studies and guidelines recommend AVF or AVG as the preferred option. Although AVF should be used whenever possible, it is crucial that clinicians consider factors such as patient size, physical exam findings, comorbidities, predicted duration of treatment to decide on the most optimal long-term vascular access modality. This article reviews the three long-term vascular access methods in children and the benefits and complications of each.     

空调换热器长效性能衰减的研究进展

空调换热器长期使用后性能会衰减,导致整机制冷和制热能力出现不同程度的下降。分别从粉尘污垢、表面腐蚀、间歇运行及微生物污染四个方面分析了国内外空调换热器长效性能衰减的研究现状,指出了目前针对这些因素的研究中存在的问题,提出了今后空调换热器长效性能的研究及应用方向。     

Changes in the physicomechanical characteristics of VT14 titanium alloy in the process of long-term loading

We study the influence of long-term loading on the behavior of physicomechanical characteristics of VT14 high-strength titanium alloy. The long-term holding of specimens (up to 21 yr) under loads close to the fracture load confirmed the stability of the long-term strength characteristics of the material. Institute of Technical Mechanics, National Academy of Sciences of Ukraine, Dnepropetrovsk, Ukraine. Translated from Problemy Prochnosti, No. 2, pp. 144–148, March–April, 2000.     

Verhalten von Gasturbinenschaufelwerkstoffen unter langzeitiger Kriechermüdungsbeanspruchung

Behaviour of gas turbine blade materials under long-term creep fatigue loading The long-term creep fatigue behaviour of the gas turbine blade material IN-738 LC HIP is investigated by a service-type strain cycle up to a test duration of about 12000 h. Further, it is reported about shorter tests of the same type on alloy IN-100 HIP. The cyclic deformation of alloy IN-738 LC HIP in the long-term region is analysed and the creep fatigue crack initiation is described by the generalized damage accumulation rule using two different methods.     

A Method for the Evaluation of the Durability of Reinforced-Concrete Structures from the Viewpoint of Fracture Mechanics

We develop a method for the evaluation of the durability of reinforced-concrete beams based on the concepts of fracture mechanics. A model of fracture of cracked structures taking into account the process of creep of concrete is constructed. We deduce dependences describing the influence of crack opening displacement on the growth of its length under long-term loading and present formulas for the crack-mouth opening displacement under short-term and long-term loads. The development of a periodic system of cracks is described for different types of concrete subjected to long-term loading.     

The Long-Term Drift of Triple-Point-of-Water Cells

As the triple point of water is of great importance for the International Temperature Scale of 1990 (ITS-90) and for the definition of the unit of thermodynamic temperature, its long-term stability has attracted a great deal of attention. In a study of long-term stability, a mystery has been uncovered. Some triple-point-of-water cells remain stable for many decades, while others decrease with increasing age of the cells, which is called long-term drift. To investigate this mystery, we used cells with different manufacture dates ranging from 1974 to 2002 and compared their analyses, which were done in 1984 and 2003. Using the same model of long-term drift as that used by Hill, the long-term drift rates of the two data sets are –4.7 μK·year⁻¹ and –9.2 μK·year⁻¹, respectively. One is consistent with the observed depression of about –4 μK·year⁻¹ measured by Hill, whereas the other differs greatly from Hill’s result. In addition, corresponding factors influencing long-term drift are discussed in this paper.     

Accuracy limitations of FOG-based continuous measurement-while-drilling surveying instruments for horizontal wells

Horizontal drilling processes in the oil industry utilize directional measurement-while-drilling (MWD) instruments to provide real-time monitoring of the position and the orientation of the bottom hole assembly (BHA). It has been reported that a single fiber optic gyroscope (FOG) can be incorporated with three-axis accelerometers to provide real-time MWD surveying of horizontal wells. However, the long-term performance and the accuracy limitations of this FOG-based instrumentation system have not been discussed. This article aims at describing a methodology for quantitative long-term analysis of the various surveying errors while drilling the near-vertical sections of the well. It also offers some techniques to enhance the long-term surveying accuracy in an experimental model of the FOG-based downhole-surveying instrument. The surveying errors are optimally estimated by the Kalman filtering techniques, and their long-term analysis is based on studying the corresponding mean square estimation errors. In order to limit the long-term growth of the surveying errors, we suggest improving the velocity computation provided by the FOG-based system either by continuous velocity update or by zero velocity update at some predetermined surveying stations. These techniques have significantly limited the long-term growth of the position errors (less than 100 m over a more than 2-h experiment). Moreover, the errors associated with the BHA orientation components were kept at less than 1/spl deg/. Suggested methodology significantly improved the surveying accuracy in an experimental model of the FOG-based MWD surveying system.     

浅谈预拌混凝土企业应收账款的管理及其对策

分析预拌混凝土行业应收账款金额长期居高不下的原因和持有应收账款的利弊,并对应收账款的管理及采取的对策提出建议。     

复合材料螺栓连接预紧力松弛的改进预测模型

该文研究了复合材料构件螺栓连接预紧力的长期性能预测。在作者前期工作的基础上,建立并完善了基于蠕变全应变理论的螺栓连接预紧力松弛预测模型。主要改进包括各蠕变阶段表达式转换为分段形式和修正时-温转换因子的作用方式两方面。采用时-温等效原理获得的长时加速表征实验数据对模型进行验证。结果表明,本模型能较好地描述预紧力长期特性,且预测效果比现有的Shivakumar-Crews模型及Hook-Norton模型更好,从而证明了该模型的准确性和可靠性。     

Hemodialysis does not impair ventricular functions over 2 years

We aimed to evaluate the long-term effect of hemodialysis (HD) treatment on left and right ventricular (LV and RV) functions in patients with end-stage renal disease. The study population consisted of 22 patients with newly diagnosed end-stage renal disease. Before an arteriovenous fistula was surgically created for HD, the patients were evaluated by echocardiography for systolic and diastolic functions. After the first HD session (mean 24.22 ± 2.14 months), the second echocardiographic evaluations were performed. Left ventricular and RV functions before and after long-term HD treatment were compared. The mean age was 55 ± 13 years and 10 (45%) of the patients were female. After long-term HD treatment, the isovolumic relaxation time was significantly decreased; however, the peak early (E) and late (A) diastolic mitral inflow velocities, E/A ratio, and deceleration time of E wave were not significantly different from the baseline measurements. Also, there was no significantly change in the early diastolic velocity (Ea) of the lateral mitral anulus and the E/Ea ratio. Pulmonary vein peak diastolic velocity, peak atrial reversal velocity, and peak atrial reversal velocity duration remained almost unchanged even though the pulmonary vein peak systolic velocity and the pulmonary vein peak systolic velocity/pulmonary vein peak diastolic velocity ratio were significantly lower after long-term HD treatment. In addition, LV systolic functions, LV diameters, LV mass index, left atrium size, and RV diastolic functions were not statistically different after long-term HD treatment. The myocardium is exposed to hemodynamic, metabolic, and neuro-humoral abnormalities during HD treatment; however, the long-term effects of HD on ventricular functions are not clearly known. The present study showed that the long-term effects of HD on LV and RV functions were insignificant in patients with end-stage renal disease. We have demonstrated that the LV and RV functions did not change significantly after long-term HD treatment. We suggest that this result may be due to regulated blood pressure levels of the patients, treatment of anemia and other metabolic disorders during the HD period and the prevention of weight gain and hypervolemia.     

Effect of fluorine incorporation on long-term stability of magnesium-containing hydroxyapatite coatings

Dissolution resistance and adhesion strength are two main concerns for long-term stability of surface coated metal implants. In this study, fluorine ions are incorporated into magnesium-containing hydroxyapatite coatings (MgF _y HA) via sol–gel method to improve the long-term stability of the implants. Surface and interface are studied in terms of phases, depth profiling and chemical bonds by grazing incidence X-ray diffraction, glow discharge optical emission spectroscopy and X-ray photoelectron spectroscopy. The long-term stability is evaluated by dissolution and pull-off test. The results show that fluorine promotes the incorporation of magnesium in HA lattice. The elemental interdiffusion and chemical bonding take place at the coating/substrate interface. Both the dissolution resistance and the adhesion strength are enhanced by fluorine incorporation, thus the long-term stability of the implant is improved.     

Shear creep of epoxy at the concrete–FRP interfaces

This paper presents the results of experimental and analytical investigations of the long-term behavior of the epoxy used at the concrete–FRP interfaces. Double shear long-term test was performed on specimens composed of concrete blocks bonded to FRP sheets using epoxy. Three test replicates were examined under sustained shear stress for up to six month time period with two primary parameters: the shear stress level and the epoxy thickness. The investigation showed that shear stress level might have significant effect on the long-term behavior of epoxy at the concrete–FRP interfaces. Based on the experimental results, creep characteristics of epoxy in the concrete–FRP interfaces were evaluated. Finite element analysis was performed incorporating these creep characteristics to investigate the long-term deformations and stress redistributions in the test specimens. It was recognized that creep of epoxy might result in stress-redistribution at the concrete–FRP interfaces.     

一种可长期维持低露点测试室的方法及其应用

研究了用5A型分子筛长期维持低露点测试室装置的原理,以及用作传感器的长期稳定度测试的方法与结果.     

Prediction of the Long-Term Strength of Various Classes of Metallic Materials

We apply the method of base diagrams to analyze more than 200 diagrams of long-term strength for dispersion-hardened nickel and aluminum alloys. By using only two close numerical values of a single constant, we attain a practically acceptable accuracy of prediction of long-term strength.     

A United States perspective on long-term management of areas contaminated with radioactive materials

The US has far-reaching and extensive experience in the long-term management of areas contaminated with radioactive materials. This experience base includes the Department of Energy's continued follow-up with Hiroshima and Nagasaki from the 1940s at the Radiological Effects Research Foundation in Hiroshima, Japan, the long-term management of the Marshall Islands Programme, the clean-up of the US nuclear weapons complex and the ongoing management of accident sites such as in Palomares, Spain. This paper discusses the lessons learnt and best practices gained from this far-reaching and extensive experience in the long-term management of areas contaminated with radioactive materials.     

The morphology,chain structure and fracture behaviour of high-density polyethylene

High-density polyethylene (HDPE) is being used more and more in critical long-term applications. For this reason it is important to have a strong understanding of those parameters which control the fracture behaviour of HDPE. In Part I of this work, fracture results were presented for eleven HDPE samples tested using a tensile testing machine. Such short-term tests do not accurately reflect the in-service loads on HDPE components, which tend to be low and static. It is, therefore, important to perform fracture tests under long-term static loads. The results of such testing are presented in this paper. The resistance to static fatigue was found to be most strongly dependent on molecular weight. Short branch concentration and short branch length were also found to exert an influence on the resistance to static fatigue. This result is similar to the findings presented in Part I of this work. However, there is some evidence that molecular weight influences fracture behaviour to a greater extent in the long-term tests. Notwithstanding, the similarity between the short-term and long-term results is important. It means that an early indication of the long-term performance of HDPE resins can be obtained from rapid comparative tests conducted using a tensile testing machine.